Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick. A substantially cylindrical core shroud is positioned within, and spaced from, the shell walls to restrain horizontal movement of the reactor core fuel bundles and provides flow separation between the downcomer and the upward core flow.
A bottom head knuckle, or junction, assembly forms an interface, or junction, between the cylindrical shell and a substantially disk shaped bottom head. Specifically, the bottom head knuckle assembly includes a shroud support subassembly and an annular forging having a top, cylindrical shaped end and a bottom, conical shaped end. The top, cylindrical shaped end of the forging is configured to be welded to one end of the RPV shell and the bottom, conical shaped end of the forging is configured to be welded to the disk shaped bottom head.
The shroud support subassembly includes a shroud support cylinder having an upper surface configured to be welded to the core shroud. An annular pump deck extends from an outer surface of the shroud support cylinder. Shroud support legs extend from the lower surface of the shroud support cylinder. The shroud support legs are welded to weld build-up pads formed on an inner surface of the annular forging.
In boiling water nuclear reactors that include recirculation pumps, which are sometimes referred to as reactor internal pumps (RIPs), RIP shaft penetrations, or accesses, are formed in the annular forging of the knuckle assembly. These penetrations generally are referred to as RIP nozzles. At each nozzle location, a RIP casing is secured to the outer surface of the forging and a pump shaft extends through the nozzle. A pump impeller is secured to the pump shaft and extends through a diffuser opening formed in the annular pump deck.
During reactor maintenance outages, the RIPs may be operated at a minimum speed for mixing vessel water. During such an outage, however, tools and other equipment in the RPV may be drawn toward and possibly into the RIP diffusers. Of course, such equipment can significantly damage the RIP. In addition, during such maintenance outages, there is a potential for objects, such as tools, underwater lights and cameras, to be dropped into a difficult to reach space between the shroud support legs and the annular forging. Removal of such objects could require draining the RPV, removal of the RIPs, and underwater searching. These operations are extremely time consuming and can extend a planned outage.
Of course, extending a planned outage to repair an RIP or remove a dropped object typically is extremely expensive. In addition, if workers are required to perform certain operations such as removal of damaged pumps, worker radiation exposure can also be increased.
It would be desirable to prevent objects from being drawn into the RIP diffuser and from falling into the space between the shroud support legs and the annular forging, during maintenance and other outages. It also would be desirable to reduce the potential for having to extend a planned outage and prevent any unnecessary worker radiation exposure.